1. Field of Invention
The present invention relates to a surface light source device of side light type. The present invention is applied to, e.g., back lighting of liquid crystal display.
2. Related Art
The surface light source device of side light type is conventionally applied to a liquid crystal display, for example, and illuminates a liquid crystal panel from its back. This arrangement is suitable for reducing thickness of the entire shape of the device.
A rod-shape light source such as a cold cathode tube is usually employed as a primary light source for the surface light source device of side light type, and is disposed on a side of a guide plate (plate-shaped guide element). Illumination light emitted from the primary light source is passed through a side end surface of the guide plate to be introduced into the guide plate. The introduced illumination light is propagated in the guide plate and during this process, light emission occurs from a major surface of the guide plate toward the liquid crystal panel.
As the guide plate employed for such a surface light source device of side light type, there are known guide plates of a type having a generally uniform plate thickness, and of a type having a plate thickness which is reduced as getting away from the primary light source. In general, the latter type emits illumination light more effectively than the former type.
FIG. 9 is an exploded perspective view showing a surface light source device of side light type, and FIG. 10 is a sectional view taken along line A--A in FIG. 9. Referring to FIGS. 9 and 10, a surface light source device of side light type 1 includes a guide plate 2, a primary light source 3 disposed on a side thereof, a reflection sheet 4, prism sheets 5 and 6 as light control members. The reflection sheet 4, the guide plate 2 and the prism sheets 5 and 6 are laminatedly arranged.
The guide plate 2 is a plate-shaped light guide member having a wedge-shaped cross section. In this example, a scattering guide material is employed as a material of the guide plate 2. The light scattering guide material comprises a matrix made of PMMA (polymethyl methacrylate) and a great number of transmissive particles uniformly dispersed in the matrix. Index of refraction of the particles is different from that of the matrix. Such a guide plate is called a light scattering guide plate.
The guide plate (light scattering guide plate) 2 includes major surfaces providing an emission surface 2C and a back surface 2B. Instead of the light scattering guide plate 2, a guide plate made of a transparent acrylic resin, for example, may be employed. If the transparent guide plate is employed, a light scattering surface is usually formed on its back surface 2B.
The primary light source 3 includes a cold cathode tube (fluorescent lamp) 7, and a reflector 8 disposed behind a back surface of the cold cathode tube 7 and having a semicircular cross section. Illumination light is supplied toward a side end surface of the guide plate 2 through an opening of the reflector 8. The reflection sheet 4 may be a sheet-shaped regular reflection member made of metal foil or the like, or a sheet-shaped light scattering member made of white PET film or the like.
Illumination light L from the primary light source 3 is introduced into the guide plate 2 through an incidence surface 2A provided by a side end surface of the guide plate 2. Illumination light L is propagated toward a distal end while illumination light L is repeatedly reflected between the emission surface 2C and the back surface 2B along which the reflection sheet 4 is disposed. During that time, illumination light L is subject to scattering effect by the particles in the light scattering guide plate 2. If the reflection sheet 4 made of a scattering member is employed, illumination light L is also subject to light diffusing effect.
As illumination light L is reflected by the slope 2B, incident angle with respect to the emission surface 2C is reduced. Reduction in the incident angle increases component which becomes equal to or lower than critical angle with respect to the emission angle, which promotes emission from the emission surface. With this characteristic, a shortage in emission light in a region far away from the primary light source 3 is avoided.
Illumination light emitted from the emission surface 2C has a light scattering nature because illumination light has experienced scattering due to the transmissive particle, or a light diffusion due to the reflection sheet 4. However, a preferential propagation direction (main propagation direction) of illumination light emitted from the light scattering guide plate 2 is inclined toward the distal end (opposite direction from the primary light source 3) with respect to the frontal direction. The light emitted from the light scattering guide plate 2 has such directivity. This nature of the light scattering guide plate 2 is called as emission directivity.
The prism sheets 5 and 6 are arranged so as to correct the emission directivity of the light scattering guide plate 2. The prism sheets 5 and 6 are formed of transparent sheet material such as polycarbonate. The prism sheet 5 is arranged so that its prism surface is opposed to the light scattering guide plate 2. The prism sheet 6 is arranged so that its prism surface turns its back to the light scattering guide plate 2.
Each of the prism surfaces includes a great number of protrusions each having a triangular cross section extended generally in parallel in one direction. The inner prism sheet 5 is oriented so that the protrusions run in parallel to the incidence surface 2A. The outer prism sheet 6 is oriented so that the protrusions run generally perpendicular to the incidence surface 2A. Slopes of these protrusions correct the main emitting direction of emitting light into a frontal direction of the emission surface 2C. A so-called both sides prism sheet in which prism sheets are formed on both sides may be used.
In general, the surface light source device of side light type employing such a wedge-shaped guide plate and the prism sheets emits the emitting light in the frontal direction more effectively than a surface light source device of side light type employing a guide plate having generally uniformed thickness.
However, in the above-described conventional device, one or some bright lines which are unfavorable for the emission surface 2C are generated as shown with symbol K in FIG. 9. The bright lines form locally thin and high luminance band, and lower the uniformity of output light. A cause of generation of bright line is that upper and lower edges of the incident surface is illuminated strongly. The illuminated edges appear as locally high luminance band on the emission surface 2C through reflection in the light scattering guide plate 2. Such a phenomenon is called as "reflective appearance" or "reflective appearance of edge".
Removing of such a phenomenon is an important matter to improve quality of output light in the surface light source device of side light type. A weak reflective appearance similar to the reflective appearance could be generated by illuminating the upper and lower edges of left and right side surfaces adjacent to the incidence surface.